Adaptive radiation, ‘taxon murk’, and the reality of early burst speciation: an example from Australia’s scincid lizards

Abstract Early burst patterns of speciation—the disproportionate concentration of speciation events early in the history of a radiating clade—are predicted under some models of adaptive radiation. Using time-calibrated phylogenetic trees, researchers have inferred evidence of an early burst for a wide range of organisms. However, the interpretation of these patterns can be fraught with controversy, because taxonomic and sampling biases—a phenomenon we refer to as ‘taxon murk’—can lead to apparent decelerations in the rate of speciation through time. Using Australia’s diverse sphenomorphine scincid lizards as a model, we tested whether multiple forms of tip-level uncertainty, including taxonomic undersampling and lag time for species recognition, could bias inference of speciation rates. To explore the impacts of taxon murk on diversification inference, we constructed a phylogenomic tree for 1941 individuals spanning 211 nominate species of sphenomorphines, including extensive sampling of intraspecific diversity. We found that the Australian sphenomorphine radiation is characterized by a robust early burst pattern that cannot be explained by uncertainty in the nature of tip units. These results are surprising, because extinction-mediated turnover should erode the signal of early burst speciation from molecular phylogenies. We provide a possible resolution to this paradox and consider the implications of our findings for continental radiations more generally. However, profound gaps in our knowledge of sphenomorphine behaviour and ecology limit our ability to test whether sphenomorphine macroevolutionary dynamics are consistent with paradigmatic patterns observed in better-studied radiations.